U.S. Pat. No. 4,010,319 issued to P. A. Levine Mar. 1, 1977, entitled "SMEAR REDUCTION IN CCD IMAGERS", and incorporated herein by reference describes the problems of transfer smear in field transfer type CCD imagers. Levine describes, as the basis for solving the transfer smear problem, the accumulation of transfer smear on a line of "null" charge packets clocked completely across the image (or A) register of the field transfer CCD imager. The resulting line of transfer-smear-descriptive charge packets is then subtractively combined with lines of charge packets descriptive of picture element (pixel) samples contaminated with transfer smear, to generate pixel samples substantially free of transfer smear. This form of transfer smear suppression works for static images or the static portions of images with some motion in them. Variants of this form of smear suppression which accommodate for the motion of isolated bright spots in the image field have been developed. See U.S. Pat. No. 4,490,744 issued Dec. 24, 1984 to P. A. Levine and entitled "SMEAR REDUCTION TECHNIQUE FOR CCD FIELD-TRANSFER IMAGER SYSTEM", for example. The problem of eliminating transfer smear in dynamic images has not heretofore been solved generally, however. The U.S. Pat. No. 4,490,744 solution does not work for two bright areas being projected into an image register charge transfer channel, for example.
U.S. Pat. No. 4,032,976 issued to P. A. Levine June 28, 1977, entitled "SMEAR REDUCTION IN CCD IMAGERS", and incorporated herein by reference describes the differences between transfer smear components originating during the most recent field transfer and during the previous field transfer, which transfer smear components contaminate pixel-descriptive charge packets stored in the field-storage (or B) register of the field transfer type of CCD imager. This patent describes how the erasure of the image register after field transfer can keep transfer smear components originating during a field transfer from contaminating the next field of image samples. This erasure does not suppress transfer smear components generated in the current field transfer, which are clocked out of the image register together with image samples. Furthermore these transfer smear components by themselves are not susceptible to suppression using a line accumulated transfer smear charge packets. Image register erasure is used as an element in the present invention, as will be described, but the invention goes further, to also solve the problem of how to suppress transfer smear components generated during the current field transfer and also to solve the problem of dark current staircase build-up during transfer.
While Levine describes an image register erasure technique applicable only to surface-channel image registers, image registers erasure techniques applicable to the buried-channel image registers now preferred are also known. They are implemented by applying adjustable voltage to gate electrodes between the image register charge transfer channels and the anti-blooming drains paralleling them. During normal operation of the image register, the voltage is adjusted to induce potential-energy barriers to charge carrier transfer from the buried channels to the drains. During erasure of the image register, the voltage is adjusted to induce surface channels between the buried channels and (anti-blooming) drains parallelling the buried channels. The clocking voltages on the gate electrodes crossing the buried channels, which gate electrodes are used to transfer charge packets forward along the buried channels, can be simultaneously made all relatively negative, to aid the transfer of all charge from the buried charge transfer channels into the drains parallelling them.
Another problem encountered in a field transfer type of imager is the accumulation of increased levels of dark current in the later lines of the field. This problem is described in U.S. Pat. No. 4,496,982 issued to P. A. Levine Jan. 29, 1985, entitled "COMPENSATION AGAINST FIELD SHADING IN VIDEO FROM FIELD TRANSFER IMAGERS" and incorporated herein by reference. See also U.S. Pat. No. 4,498,105 issued to D. D. Crawshaw Feb. 5, 1985 and entitled "FIELD-TRANSFER CCD IMAGERS WITH REFERENCE-BLACK-LEVEL GENERATION CAPABILITY". Prior-art approaches to solving the dark current ramp problem accumulate dark current at sites proximate to the field storage (or B) register, from which accumulated dark current compensating offsets for each successive line of pixel samples are derived. The problem with these approaches is that there are variations in the dark current components accompanying pixel samples along each line of pixel samples. These variations are due to localized heating effects in the B register, due to differences in the semiconductor device structure in different parts of the B register, due to defects in the semiconductor device structure within the B register, or due to combinations of these effects. These variations in dark current are not compensated against in prior art dark-current ramp suppression schemes.